1. Field of the Invention
The present invention relates to a quadrature demodulator that creates an I-signal and a Q-signal with a baseband to demodulate a received signal, and an interrogator comprising the quadrature demodulator.
2. Description of the Related Art
An interrogator is a radio communication device that makes radio communications with a transponder called a radio tag or a radio-frequency identification (RFID) tag. The interrogator uses a modulated radio signal to transmit information to the RFID tag, and after the transmission of the information, continues to transmit an unmodulated signal. In contrast, the RFID tag changes the amount of reflection in the unmodulated signal from the interrogator for backscatter modulation and transmits the resultant information to the interrogator. The interrogator receives the backscatter modulation wave to read the information from the RFID tag.
The interrogator comprises a transmission section and a reception section. On the transmission side, a modulator modulates the information, and an amplifier amplifies the modulated information and then transmits the information from an antenna. On the reception side, the antenna receives the signal, and a direct-conversion quadrature demodulator extracts a baseband signal from the high-frequency signal, and demodulates the baseband signal to extract information from the signal.
The direct-conversion quadrature demodulator inputs the reception signal and a local signal of the same frequency as that of a carrier for the reception signal, to a mixer to create an in-phase (I) signal with the baseband. The direct-conversion quadrature demodulator inputs the reception signal and a signal out of phase with the local signal by 90 degrees, to the mixer to create a quadrature-phase (Q) signal with the baseband.
The amplitudes of the I- and Q-signals depend on the phase difference between reception signal and the local signal. The amplitude of the Q-signal is minimized by maximizing the amplitude of the I-signal. The amplitude of the Q-signal is maximized by minimizing the amplitude of the I-signal. When the Q-signal has the minimum amplitude of zero, the I-signal has the maximum amplitude, and reception data can thus be reproduced using this I-signal. In contrast, when the I-signal has the minimum amplitude of zero, the Q-signal has the maximum amplitude, and the reception data can thus be reproduced using this Q-signal. The phases of the I- and Q-signals may be inverted depending on the phase difference between the reception signal and the local signal.
As a method of reproducing the reception data using such a direct-conversion quadrature demodulator, a method is known which reproduces the reception data by selecting one of the I- and Q-signals which has a greater amplitude (see, for example, U.S. Pat. No. 6,501,807 B1).
The reproduction method described in U.S. Pat. No. 6,501,807 B1 reproduces the reception data by comparing the amplitudes of the I- and Q-signals with each other to select one of the I- and Q-signals which has the greater amplitude. Thus, when the amplitude of the I-signal is markedly different from that of the Q-signal, the reproduction is prevented from being affected because the selected signal has the large amplitude. However, if the I- and Q-signals have almost the same amplitude, either of the signals may be selected, but the reception data needs to be reproduced at an amplitude that is half that of the reception signal. Thus, when the reception signal is at a low level, the signal is likely to be affected by noise. Thus, disadvantageously, the reception data is often erroneously reproduced by the noise.